The microflora of the human large intestine plays a crucial role in human health. The microflora can be modulated by dietary intake. Undigested carbohydrates and undigested proteins are metabolised by the bacteria and as a major end-product of metabolism short-chain fatty acids (SCFA), such as acetate, propionate, butyrate and valerate, are formed, which can subsequently be used by the host. Other examples of end products of bacterial fermentation are lactate and gas.
Lactobacilli produce either lactate or lactate plus acetate. Bifidobacteria produce lactate and acetate. Fermentation of carbohydrates by Bifidobacteria, Lactobacilli and other lactic acid bacteria usually does not lead to the production of propionate, butyrate, isobutyrate, valerate and isovalerate. These SCFA's are indicative of the fermentation of carbohydrates by other bacterial species, such as Clostridia Bacteroides or Enterobacteriaea, or are indicative for the fermentation of proteins. Therefore, a (relative) increase of intestinal lactate and acetate concomitant with a (relative) decrease of other SCFA's indicates specific stimulation of lactic acid bacteria and/or Bifidobacteria. 
Mothers' milk appears also to have a bifidogenic effect, as the dominant bacteria that become established in breast-fed infants are Bifidobacteria. In contrast, bacterial colonization of infant formula fed infants is not or less dominated by Bifidobacteria and is more diverse in bacterial species (Harmsen et al. 2000, J Pediatr Gastroenterol Nutr 30, 61-67). It is thought that non-digestible saccharides present in human milk are responsible for this bifidogenic effect. In the colon and faeces of breast-fed infants the predominant SCFA found is acetate. Furthermore high concentrations of lactate are found.
Several compositions have been proposed in an attempt to mimic the SCFA production profile of breast fed infants. Combinations of small, indigestible oligosaccharides or combinations of small, indigestible oligosaccharides with indigestible polysaccharides such as described in WO0008948 have been proposed. However, a main disadvantage of the use of small, prebiotic oligosaccharides is that it significantly increases the osmotic load in the intestine, which may result in deleterious laxative effects or even worse, diarrhoea.
Using indigestible, polysaccharides with a high molecular weight or a high degree of polymerisation (DP) has the disadvantage that the viscosity of the product is increased. Furthermore, large polysaccharides, with a DP of above 300, are more difficult to ferment by lactic acid bacteria and Bifidobacteria. 
Therefore, there is a need for nutritional formula comprising prebiotics composed of relatively large, indigestible polysaccharides, which have the desired effect on short chain fatty acid production in the intestine and which does not excessively increase intestinal osmotic load and/or product viscosity.